The present invention meets those needs by providing methods of forming polymer-ceramic composite electrolytes in which the resulting electrolyte exhibits enhanced conductivity. In one method, the electrolyte is mechanically stretched to achieve enhanced conductivity. We have found that the effect of mechanical stretching on conductivity is permanent and stable even after long isothermal stabilization. In another method, the polymer-ceramic composite electrolyte is formed by a melt-casting method which results in enhanced conductivity. The resulting polymer-ceramic composite electrolytes may be used in a variety of electrochemical applications, particularly lithium rechargeable batteries.
According to one aspect of the present invention, a method for enhancing conductivity of a polymer-ceramic composite electrolyte is provided comprising uniaxially stretching the polymer-ceramic composite electrolyte. Preferably, the polymer-ceramic composite electrolyte is stretched at a temperature of about 45 to 55xc2x0 C. The polymer-ceramic composite electrolyte is preferably in the form of a thin film which is stretched from about 5 to 15% in length. The film is preferably about 1 to 100 xcexcm thick.
After stretching, the film has a room temperature conductivity of the order of about 10xe2x88x926 S cmxe2x88x921 to 10xe2x88x924 S cmxe2x88x921. By room temperature conductivity, it is meant that the film exhibits high conductivity at temperatures ranging from about xe2x88x9240xc2x0 to 40xc2x0 C.
In one embodiment of the invention, the polymer-ceramic composite electrolyte comprises from about 30 to 70% by weight poly(ethylene oxide) (PEO), from about 10 to 20% by weight of a lithium compound selected from lithium tetrafluoroborate (LiBF4) or lithium methyl fluorosulfonate (LiCF3SO3), and from about 0.1 to 40% by weight of a ceramic material selected from the group consisting of BaTiO3, TiO2, MgO, ZnO, SrO, BaO, CaO, ZrO2, Al2O3, SiO2, SiC, Si3N4, and BN. More preferably, the electrolyte comprises from about 5 to 25% by weight of the ceramic material, and most preferably, about 20% by weight. The ceramic material preferably has an average particle size of about 5 to 100 nm.
In a preferred embodiment of the invention, the film is preferably annealed after stretching such that it has a room temperature conductivity of the order of about 10xe2x88x924 S cmxe2x88x921 to 10xe2x88x923 S cmxe2x88x921.
In an alternative embodiment of the invention, a method of enhancing conductivity of a polymer-ceramic electrolyte is provided which includes providing an amount of poly(ethylene oxide), a lithium compound selected from lithium tetrafluoroborate and lithium methyl fluorosulfonate, and a ceramic material to form a mixture, melting the mixture, and forming the mixture into a polymer-ceramic electrolyte film. Preferably, the film is also uniaxially stretched to result in a room temperature conductivity of 10xe2x88x924 S cmxe2x88x921 to 10xe2x88x923 S cmxe2x88x921. The stretched film is also preferably annealed to further enhance conductivity.
The polymer-ceramic composite electrolytes formed by the methods of the present invention have been found to exhibit excellent conductivity, and they may be effectively used in lithium rechargeable batteries and other electrochemical devices.
Accordingly, it is a feature of the present invention to provide methods of forming polymer-ceramic composite electrolytes for use in lithium batteries having enhanced conductivity. This, and other features and advantages of the present invention will become apparent from the following detailed description, the accompanying drawings, and the appended claims.